The present invention relates to a control system for a printer and is especially useful for controlling the printing of data by a rotary drum printer.
Prior art communication systems, for example controlling the environment within a building or fire and security conditions within a building, having a central panel for providing alarm indications or indications of the status of various points within the building, originally provided a separate wire running from the panel to each point which was being monitored. In today's large buildings, however, the costs of wiring in this manner can be prohibitive. Therefore, the prior art replaced these plurality of wires by a single transmission channel running from the central processing unit to the remote points being monitored.
The central processing unit is capable of scanning the status of the various remote points and controlling various operations at the remote points by addressing in turn each remote point and requiring the remote points to take some action. The action may be to report back its present status, such as temperature, pressure or humidity, or to perform some operation such as starting or stopping a fan, opening a damper or whatever. By using a single transmission channel, the cost of installation of these monitoring systems is greatly reduced.
Such communication systems must perform a variety of services other than scanning and performing operations at remote points. Indications of the status of the remote points must be given and recorded which function is often performed by printers. In a typical system the printer will print out any changes in status of the remote points. It is also desirable for the printer to print logs of the status or occurrences at the remote points. In the present invention, the printer has access to the central processor to request the transmission of certain data so that the printer can print it. The printer used in the present invention is a rotary drum printer.
Such rotary drum printers have a printing drum with characters arranged in a plurality of columns and rows. Paper is fed between this drum and a plurality of firing hammers, each hammer associated with a corresponding column of characters and the printing operation is accomplished by firing a hammer which strikes the paper to the character leaving imprinted on the paper the character on the drum associated with that hammer.
Data to be printed which is transmitted by a central processor typically comprises a plurality of bits and the bits are divided into groups of four or five, each group representing a character to be printed. The data is transmitted, usually, a line at a time. A plurality of bits, comprising a plurality of characters, are supplied to the printer which stores them and then pulls out the data bits, a character at a time, for comparison with the present drum position of the printer. Each character to be printed corresponds to a particular character in a particular column on the drum. If the drum presents its first row of characters to the hammers, only certain stored characters will correspond to the characters in that row. Thus, only those hammers relating to the characters in the row which match the stored characters will be fired.
The drum revolves to a new row after the hammers have been fired at the first row. The stored characters are compared with the new drum position and new corresponding hammers are fired. This operation continues until all rows have been compared with the stored characters. As the last row is compared, the entire line will have been printed and the printer can print a new line of data from the central processor.
Since all data characters must be compared with each drum position and since only a character at a time is compared, some means must be provided for keeping any matches until all stored characters have been compared with the drum row position at which time all hammers corresponding to the matches may be fired in unison. The prior art apparatus have typically used a series of gates connected to the output of the comparator and to a distributor for selecting which gate is to pass the comparator output and a series of latches, each associated with a column on the printer drum for storing the output from the comparator through the series of gates. As can be seen, this prior art apparatus is extremely complex requiring the employment of a considerable amount of logic circuitry.
The present invention simplifies the control apparatus and reduces the amount of logic circuitry needed by shifting the output from the comparator through a shift register which automatically keeps track of the data characters compared by the comparator and their associated column position on the printer drum. When all of the data characters have been compared with the drum position, a signal is generated which allows the output from the shift register to be transmitted to the firing circuitry for firing the associated hammers of the printer.
In addition, the prior art systems have not provided a printing control system which can both receive data to be printed and, while data is being received, print data which has previously been received in a manner as simple as that provided by the instant invention. To accomplish this operation, the present invention uses a random access memory with a write addressor for storing incoming data in one half of the RAM and a read addressor for reading out and printing data stored in the other half of the RAM. The control system includes a mechanism for controlling the highest order storage bit location of the RAM which divides the RAM in half so that one half can store data and the other half can provide data to the printer for printing.